In an effort to maximize the efficiency of commercial vehicles used to transport goods, such vehicles have been made increasingly larger in size and carrying capacity. For some applications, it is desirable to pull a tandem trailer, which is usually as long as two trailers, with a single vehicle to further increase the carrying capacity of the vehicle. However, the advantage of having a single operator deliver a large quantity of goods, whether measured in number or by weight, is accompanied by more difficult vehicle maneuverability which in turn requires an operator of greater skill and experience.
Tasks which are relatively simple to perform by the operator of a passenger vehicle, such as turning a corner or backing the vehicle, become significantly more cumbersome and require greater skill and attention when pulling a trailer, especially a fully loaded semi-trailer. Since the trailer wheels do not follow the same path as the wheels of the tractor (a condition known as off-tracking), the operator must alter the path of the tractor to compensate accordingly.
The amount of compensation required for the trailer wheels to follow all acceptable path which is clear of obstructions such as curbs and utility poles, is largely dependent upon the length of the trailer. The longer the trailer, the more compensation required. The wheel path compensation required makes the vehicle considerably more difficult to maneuver, especially around sharp corners.
Maneuverability can be greatly improved by providing a steerable trailer axle. One, somewhat dated, but prominent display of this solution is illustrated by the traditional hook and ladder type fire engine. Such vehicles employ one vehicle operator to steer the towing vehicle and another operator located at the rear of the trailer to steer the trailer axle. Thus, successful navigation of the vehicle requires two skilled operators acting in concert.
More recently, systems have been developed which incorporate a steerable trailer axle which is automatically controlled by an on-board electronic control unit. These systems substantially eliminate the off-tracking of the trailer wheels characteristic of conventional tractor-trailer vehicles without the need for compensation by a skilled operator. However, for proper operation, such systems require an accurate estimate of the length of the trailer being towed.
For proper trailer wheel tracking in a typical tractor semi-trailer application, the estimate of the trailer length should accurately gauge the distance between the center of the tandem axle on the tractor and the center of the tandem axle on the trailer. The overall trailer length estimate can be less accurate than this more critical dimension unless the overall length is required for use by a controller or routine which assists the operator in backing the vehicle. Thus, it is desirable to have a system capable of accurately estimating the length of the trailer and, more precisely, the distance from the rear axle of the towing vehicle to the trailer axle.
Trailer length is also used by vehicles equipped with electronically controlled braking systems. The length of the trailer is an important parameter used by tractor-based trailer stability control systems to differentiate between a jackknife condition in which the trailer is pushing the tractor, and the more common, controlled turning condition., so that the appropriate action, if any, may be taken. If an impending jackknife condition is detected, appropriate action may include operating the braking system of the tractor, the trailer, or both.
The prior art discloses a number of devices which assist the operator of a vehicle towing a trailer in performing tasks such as backing the vehicle and safely passing other vehicles. These devices, such as those disclosed in U.S. Pat. No. 4,645,315 to Morgavo; U.S. Pat. No. 4,970,496 to Kirkpatrick; and U.S. Pat. No. 5,196,965 to Lang use apparatus mounted toward the rear of the trailer to sense the distance, or assist the operator in sensing the distance, between the trailer and some other object. However, none discloses a system or method for actually determining the length of the trailer to be used in similar type applications or for use in a vehicle towing a trailer with a steerable axle to eliminate off-tracking.